This proposal focuses attention on the influence of aging gametes on problems of fertility, defective embryos, and embryo mortality. Also, the excellent embryo culture techniques available make feasible critical examination of the conditions required for suitable storage of embryos, and the influence of the uterine environment at the time of fertilization on the probability of normal development. Specifically planned with rabbits and/or cattle are investigations in the following areas: 1. Post-ejaculatory deterioration of spermatozoa, especially the biochemical changes which occur during preservation by cryogenic means and are associated with fertilization failure and embryo development, will be studied. Possible means of prevention will be examined simultaneously. 2. The embryo culture technique will be modified as the basis for protecting and preserving embryos by cryogenic means, thus extending the usefulness of embryo culture beyond the laboratory into more areas of clinical importance. 3. The influence of hormone imbalance at conception on subsequent embryo failure will be explored further.